DK151342B - PROCEDURE FOR REMOVAL OF RESIDUAL VINYL CHLORIDE MONOMER FROM A VINYL CHLORIDE POLYMER - Google Patents

Description

1 151342

The present invention relates to a process of the kind set forth in the preamble of claim 1 for removing residual vinyl chloride monomer from a vinyl chloride polymer obtained by polymerization in aqueous suspension or emulsion, and the process of the invention is characterized by the characterization of claim 1. part stated.

Polymerization in suspension or in aqueous emulsion is a frequently used technique for preparing vinyl chloride polymers. It is common practice in this technique to stop polymerization after achieving a conversion rate of the order of 90-95%. Obtaining higher conversion rates close to 100% implies a significant extension of the duration of the polymerization process and, moreover, causes a certain risk of degradation of the polymer.

Despite the degassing at the end of polymerization, which is usually done to evaporate unreacted vinyl chloride, the aqueous dispersions of vinyl chloride polymers thus obtained still contain significant amounts of unpolymerized monomer.

Usually the suction of the polymer is followed by drying of the suctioned cakes, thus further eliminating some of the residual monomer. However, the dry vinyl chloride polymers thus obtained contain relatively large amounts of residual vinyl chloride monomer.

Various means have been proposed to reduce the residual monomer content in vinyl polymers, but these agents have proved to be poorly effective.

However, Belgian Patent No. 793,503 discloses an effective method of removing the residual vinyl chloride present in solid vinyl chloride polymers, thereby heating the solid polymer to a temperature between its glass transition temperature and the temperature of initial decomposition by condense water vapor on the polymer and keep the polymer at this temperature for a sufficient period of time to eliminate most of the monomer (s) present in the polymer, and then cool it to a temperature below the glass transition temperature by evaporating the condensed water vapor at polymer to recover a dry polymer free of residual monomer.

However, this patent does not mention the possibility of applying the above-described treatment to a polymer present in the form of an aqueous dispersion, in particular an aqueous dispersion derived directly from the polymerization.

An effective method has now been found to eliminate residual vinyl chloride from vinyl chloride polymers present in aqueous dispersion, which allows treatment of the aqueous dispersions derived directly from the polymerization, thereby avoiding any release of vinyl chloride into the environment. .

The invention thus relates to a method for removing residual vinyl chloride monomer from a vinyl chloride polymer containing more than 50 mole% of monomer units derived from vinyl chloride by heating to a temperature at least equal to the glass transition temperature of the polymer and treating with steam at this temperature.

The process of the invention is characterized by heating the aqueous polymer dispersion obtained by the polymerization directly to a temperature at least equal to the glass transition temperature of the polymer and subjecting it to entrapment with steam while maintaining a temperature which is at least equal to the glass transition temperature of the polymer.

The process of the invention can be applied to all aqueous dispersions of vinyl chloride polymers which originate directly from the polymerization, e.g. can be carried out by suspension or emulsion processes. In that case, the aqueous dispersion using the process of the invention can contain, without disadvantage, the usually-present additives in the polymerization, such as initiator residues (organic or inorganic peroxides or azo derivatives), emulsifiers (by polymerization in emulsion), dispersants (by polymerization). in suspension) etc. These dispersions usually contain approx. 5-10 parts by weight 3 vinyl chloride per 100 parts polymer. The process according to the invention is particularly well suited for the treatment of aqueous dispersions obtained by suspension polymerization. In aqueous dispersions obtained by emulsion polymerization, the fineness of the particles and the presence of emulsifiers can induce foaming phenomena and entrapment of particles, making the application of the process more difficult.

Since the process of the invention is applied to aqueous dispersions derived directly from the polymerization, it is possible to pre-modify the solids content of the dispersion by diluting or concentrating it, or to modify the physical state of the dispersion, e.g. by coagulating it. A pre-degassing treatment may also be performed to remove a substantial portion of the vinyl chloride present in advance.

This degassing is carried out in a manner known per se at the end of the polymerization and before the treatment according to the invention by lowering the pressure, e.g. to atmospheric pressure, and then subject the aqueous polymer dispersion to a partial vacuum. These operations are generally accompanied by cooling of the dispersion on the order of 5 - 15 ° C, which is primarily due to the evaporation of part of the residual vinyl chloride.

The process of the invention can, as mentioned, be applied to all vinyl chloride polymers containing more than 50 mole percent of monomer units derived from vinyl chloride, and preferably those containing more than 70 mole percent of such units. Thus, the process can be applied to vinyl chloride homopolymers, statistical copolymers, graft copolymers and solid products containing such polymers. It goes without saying that the process of the invention is also effective in eliminating the residues of volatile comonomers that may be present in the aqueous dispersions.

To determine the polymer temperature for transition to the glass state, a differential thermal analysis is advantageously used which provides a sufficiently accurate measurement of the glass transition temperature.

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The means used for carrying out the first step of the process according to the invention, which consists in bringing the aqueous polymer dispersion up to a temperature higher than the glass transition temperature, may be arbitrary. heat the aqueous dispersion by circulating a hot fluid through the dual jacket to the container in which the dispersion is located, and / or directly inject an inert hot fluid such as air, nitrogen or water vapor. However, it is preferred to introduce water vapor to preheat the aqueous dispersion. The use of water vapor thus enables one to reach the desired temperature in a very short period of time.

The second step of the process according to the invention consists in entraining the residual monomer by an inert fluid while maintaining the aqueous dispersion at a temperature at least equal to the glass transition temperature.

Since the entrainment is carried out by steam, the subsequent separation of the vinyl chloride from this entrainment fluid is simplified. For this purpose, water vapor may be introduced into the aqueous dispersion, e.g. superheated water vapor, which is blown in sufficient quantity to ensure entrainment of released vinyl chloride, maintaining the desired temperature and stirring the medium. It is also possible to generate the necessary water vapor for entrainment by heating the aqueous dispersion sufficiently to bring it to a boil. This heating can be done through the walls of the container in which you are working or otherwise.

During this second step can. one also continues to supply the necessary calories in whole or in part through the double container sheath or otherwise.

As for the temperature of the aqueous dispersion during the entrainment step, it is essential to keep it at a value at least equal to the glass transition temperature of the polymer. The upper limit of the temperature at which the entrainment can be effected depends on the decomposition temperature of the polymer. Optionally, a thermal stabilizer may be added.

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However, it should be noted that heating to high temperatures substantially above 100 ° C represents an unnecessary energy consumption. Moreover, the aqueous dispersion treated at a high temperature must later be cooled in order to be subjected to suction, which increases the duration of the cooling and / or the required volume of the refrigerant.

For these reasons, it is preferable to heat the aqueous dispersion to a temperature not exceeding 140 ° C and preferably not exceeding 120 ° C.

The pressure prevailing in the container in which the removal of the residual monomer takes place is controlled as a function of the selected temperature.

Preferably, saturated water vapor pressure is used under the working conditions because water vapor is used as the entraining fluid. Thus, a convenient method is provided for controlling the temperature of the aqueous dispersion.

Nor does the duration of entrainment constitute a critical parameter in the method of the invention. It is a function not only of the temperature to which the aqueous dispersion is heated, but also of its original residual monomer content, the desired degree of removal, and the porosity of the polymer.

As a result, the duration of entrainment can be easily determined experimentally in each case. As a guide, from a few minutes to approx. 2 hours, especially 5-45 minutes, will generally be sufficient time to reduce the residual monomer content in the aqueous vinyl chloride polymer dispersions to a few tenths of ppm. expressed as the weight relative to the polymer.

The amount of entrainment fluid is also advantageously determined experimentally in each case. It goes without saying that the efficiency of entrainment increases up to a certain limit, while increasing the amount of inert fluid leaving the aqueous dispersion. This efficiency also increases with temperature, which, when operated at a relatively high temperature, allows a reduction in entrainment time. In this regard, a temperature range that gives particularly good results is in the range of 90 to 110 ° C. Working in this area can reduce the entrainment time to less than 15 minutes, while eliminating the remaining vinyl chloride almost completely.

Furthermore, it has been found that one can further significantly improve the efficiency of the method according to the invention.

For this purpose, the treatment is continued after heating the aqueous polymer dispersion to a temperature at least equal to the glass transition temperature of the polymer and then subjecting it to entrainment with an inert fluid while maintaining a temperature at least equal to glass transition temperature, lowering the aqueous dispersion to boil.

Thus, the process of the invention may comprise an additional third step of entraining water vapor which runs under the action of the water vapor which is developed by boiling the aqueous dispersion. The aqueous dispersion is at the beginning of the third step at a temperature at least equal to the glass transition temperature of the polymer. The pressure is lowered so that it is lower than the saturation vapor pressure of water vapor at the aqueous dispersion temperature. During this step, the removal of vinyl chloride is thus completed. This third step allows for a substantial reduction in the duration of the previous step as well as the heat consumption during the previous step. During the boiling of the aqueous dispersion, one can of course continue to heat the aqueous dispersion and allow an inert entraining fluid to bubble through. However, it is preferred that the aqueous dispersion be cooled progressively during this third step until a temperature below the glass transition temperature of the polymer. For this reason, it is preferable to limit and even completely exclude heat supply from outside during the third step. Therefore, as the aqueous dispersion cools, the pressure should be progressively lowered so that the cooking is maintained. This approach has been found to be particularly effective. Furthermore, the total treatment time is thereby shortened by accelerating the cooling of the aqueous dispersion.

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The time at which the third step begins is selected as a function of the final residual monomer content that one wishes to obtain. This can easily be determined experimentally. As for the duration of the third step, this varies as a function of the operating conditions, but is usually 5-45 minutes.

After the residual monomer removal treatment, the polymer is separated from the aqueous phase. This separation is usually done in two steps: In the first step, the majority of the water is separated to obtain a moist cake, e.g. by filtration or centrifugation on which the polymer is dried, e.g. by fluidization. Since the apparatus used to liberate the polymer from a significant portion of the water phase (first stage) is generally not adapted to operate at temperatures near the glass transition temperature, the aqueous dispersion is cooled before removing most of the water. to a temperature lower than the glass transition temperature of the polymer, e.g. to 50 - 80 ° C, in particular approx. 70 ° C, preferably by lowering the pressure as described above. Since the polymer must be heated during drying, it is not advisable to go below these temperatures for economic reasons.

The heating of the aqueous dispersion and entrainment with steam can be carried out in the polymerization reactor or in a special container for this purpose. The preliminary degassing and final boiling can optionally be carried out in the same container as the process itself. It is preferred to carry out the method in a special container provided with a double jacket through which a heating fluid can be circulated.

It is also advantageous to provide this container with a stirring means, e.g. a bucket stirrer.

Independent of the selected container for carrying out entrainment of residual monomer with vapor, i.e. the polymerization reaction itself or a specially designed container for this purpose, it is particularly advantageous to introduce the steam at the bottom of the treatment vessel, e.g. by means of an inlet pipe protruding into the aqueous dispersion, or through B 151342 a belt valve. In this way, an efficient flow of steam through the aqueous dispersion is ensured.

The process can be carried out continuously or discontinuously. As the polymerization is generally carried out discontinuously, it is also preferred to carry out the process according to the invention discontinuously.

In a preferred embodiment of the process according to the invention, the condensable compounds, in particular water vapor contained in the gaseous effluent leaving the aqueous dispersion during the process, are condensed by means arranged prior to the gaseous evacuation system. effluent. For this purpose, a capacitor may be inserted between the container in which the treatment is taking place and the means which maintain the desired pressure in the container. In practice, apart from the pressure losses, the capacitor operates at the same pressure as the container. However, it can be made to operate at a lower pressure by inserting a valve between the container and the capacitor.

When the treatment does not involve the use of temperatures above 100 ° C, the pressure in the container is at most equal to atmospheric pressure. In this case, a vacuum pump is used to maintain the desired pressure in the container.

When the treatment comprises phases carried out at a temperature higher than 100 ° C, these should be carried out under pressure. Nevertheless, it is desirable to have an apparatus for establishing a vacuum in the container, especially when employing the preferred variant of the process of the invention, which consists in terminating the treatment with a boiling of the aqueous dispersion induced by a pressure drop.

You can use all the usual types of vacuum pumps. Thus, e.g. use ejector pumps. However, since ejector pumps consume a large amount of energy, it is preferred to use rotary pumps, such as liquid ring pumps.

It goes without saying that the treatment according to the invention can be carried out in installations where a vacuum pump is directly connected to the treatment vessel. In this case, a capacitor can be placed after the vacuum pump. However, this arrangement has been found to have serious drawbacks. Thus, if an ejector pump is used as a vacuum pump, its consumption of water vapor is absolutely prohibitive because of the large volume of vapors produced during the treatment. Furthermore, the total vapor volume, which is extremely large, requires the use of a very large volume capacitor. If a rotary pump is used as a vacuum pump, frequent interruptions and rapid destruction of the pump can be observed due to the high temperatures of the evaporated vapors and their chemical nature.

It has been found that these problems can be easily solved by using a capacitor located between the treatment vessel and the vacuum pump. Preferably, an exchange type condenser is used which is dimensioned so that during treatment, a very substantial portion of the water vapor leaving the treatment vessel can be condensed.

In order to prevent the polymer particles from being entrained to the capacitor, a foam separator may be placed between the treatment vessel and the capacitor, e.g. a hydrocyclone which stops the entraining particles which may be recycled to the treatment vessel by means of a stream of water.

The condensed water can be fully or partially reintroduced into the aqueous dispersion.

The condensation of the gaseous effluent prior to the vacuum system allows the use of low capacity vacuum pumps and ordinary construction, without interruptions being frequent.

Finally, by recycling the condensed water vapor to the aqueous dispersion, any risk of draining water containing traces of vinyl chloride is avoided.

The aqueous polymer dispersions obtained after the treatment contain practically no residual vinyl chloride. leafy, the removed monomer from the aqueous dispersions can be easily recovered by a simple condensation of the water vapor.

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Thus, the process of the invention provides several significant advantages, namely obtaining vinyl chloride polymers with a very low residual monomer content, a simple and economical recovery of removed residual monomer, and a very significant reduction in the amount of monomer emitted in the atmosphere or in the surface water. .

It has further been found, contrary to what might be feared, that the treatment of the aqueous suspensions of vinyl chloride polymers by the process of the invention does not cause any degradation of the said polymers. On the contrary, the treated vinyl chloride polymers after the treatment have a considerably improved thermal stability and their thermal stability over a long period of time is in no way affected by the treatment according to the process according to the invention.

In short, the process of the invention not only allows for the removal of the vinyl chloride located on the polymer particles in good yield, but also for the removal of the vinyl chloride collected in the interior of the particles. Under well-chosen operating conditions, the dried polymer contains less than 2 ppm residual vinyl chloride monomer, making it suitable for use in the manufacture of food packaging such as bottles.

The invention is further illustrated by the following examples.

4 In Examples 1, 2, R, R and 7, an aqueous polyvinyl chloride dispersion obtained by polymerizing vinyl chloride in aqueous suspension at 70 ° C with degassing at 60 ° C under partial vacuum (absolute residual pressure 350 mmHg) was used.

Its polyvinyl chloride has the following characteristics:

- glass transition temperature: 88.5 ° C

- porosity (by absorption of dioctyl phthalate): 11% - apparent density (by compression): 0.66 kg / dm

C

In Examples 5cgR °, an aqueous polyvinyl chloride dispersion obtained by polymerizing vinyl chloride in aqueous suspension at 60 ° C was used with degassing at 55 ° C under partial vacuum (absolute residual pressure 350 mmHg).

This polyvinyl chloride has the following characteristics:

- glass transition temperature: 89 ° C

- porosity (by absorption of dioctyl phthalate): 21% - apparent density (by compression): 0.55kg / dm3.

EXAMPLE 1 In a 16 1 container connected to a vacuum pump and fitted with a double jacket, a stirrer and a steam supply tube which protrudes into the container, with stirring 10 1 aqueous suspension is introduced, after establishing an absolute residual pressure of 526 mmHg, superheated water vapor at 152 ° C is introduced into the aqueous dispersion through the feed tube (amount of 4kg / H). The aqueous dispersion, which initially is at ca. 60 ° C, is rapidly heated in contact with the condensing water vapor. When the aqueous dispersion has reached a temperature of 90 ° C, i.e. the temperature corresponding to the dew point of the water vapor at the operating pressure, the water vapor practically no longer condenses and is eliminated directly through the vacuum system while entrapping residual vinyl chloride. The superheated water vapor is continued to be injected into the aqueous dispersion, maintaining the temperature of 90 ° C for 30 minutes (amount of 1 kg / h). After this time, the steam vapor and the vacuum pump are disconnected and cold water is circulated for circulation in the double jacket, whereupon the suspension is cooled to 70 ° C.

Water vapor and entrained vinyl chloride are separated by condensation of the water vapor.

The initial content and final content of vinyl chloride in the aqueous suspension are shown in the table below.

The cooled aqueous suspension is aspirated and the cake is dried for 2 hours at 65 ° C. The content of vinyl chloride in the dry polyvinyl chloride is also shown in the table.

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The thermal stability is assessed on a product prepared as follows: Incorporate 20 g of dry resin, 20 g of dioctyl phthalate, 1.2 g of a barium-cadmium stabilizer and 1 g of stearic acid.

After mixing the product in a cylinder tent for 2 minutes at 175 ° C, a crepe-like, almost unstained product (very pale pink) is obtained.

EXAMPLE 2

This example is identical to the above except that the water vapor drive is carried out at 100 ° C for 15 minutes.

In this case too, the initial coloring of the crepe-like product was very pale pink.

The operating conditions and results are shown in the table below.

Samenligningseksempel_R_

This example relates to treatment of an aqueous polyvinyl chloride suspension identical to that used in Example 1, however, the treatment is carried out at 80 ° C, i.e. below the glass transition temperature of the polymer.

Experimental conditions and results are shown in the table.

A

Example Example_R_ In this example, the classical treatment of an aqueous suspension of polyvinyl chloride is identical to the one used in Example 1, i.e. degassing, suction and drying at 65 ° C for 2 hours.

The conditions and results are shown in the table.

In this case, the initial staining of a crepe-like product prepared under the same conditions as in Example 1 was pronounced pink.

EXAMPLE 5

This example is identical to Example 1, but only for a duration of 15 minutes. Further, the cake is dried at 70 ° C for 2 hours.

The conditions and results are shown in the table.

The initial staining of a product prepared as in Example 1 is very pale pink.

c

Comparative Example In this example, the classical treatment of an aqueous polyvinyl chloride suspension is carried out as in Example 5. However, the degassed suspension is suctioned off and dried for 2 hours at 70 ° C.

The initial staining of a product prepared as in Example 1 was pronounced pink.

EXAMPLE 7

This example illustrates the completion of entrainment of residual water vapor monomer produced solely by boiling the aqueous suspension, as well as the use of condenser. 1 2 3 4 a 16 1 container which, through an exchange-cooled condenser of the exchange type with a exchange surface of 400 cm, is connected to a vacuum pump and provided with a stirrer and a steam supply pipe which is introduced into the container, during stirring and after establishment of an absolute residual pressure of 526 mmHg 10 l degassed aqueous suspension. Subsequently, 3 through the feed tube superheated water vapor is introduced at 152 ° C (quantity: 4 kg / h). The dispersion, which initially is at approx. 60 ° C, is quickly heated in contact with the condensing water vapor. During this heating step, the cooling water does not circulate in the condenser. When the aqueous dispersion has reached a temperature of 90 ° C, i.e. at a temperature corresponding to the dew point of the steam at the operating pressure, the water vapor practically no longer condenses and entrainment with the water vapor begins. From this point on, the amount of water vapor is reduced to 1 kg / hour and the condenser started, while regulating the amount of gaseous effluent that is extracted so that the temperature is kept constant for 20 minutes. After this time, and after taking a sample of the aqueous suspension, the supply of superheated water vapor is discontinued and the vacuum pump is kept running while allowing the capacitor to operate at maximum capacity. The aqueous dispersion is thus kept under boiling with decreasing temperature for 10 minutes. After this time, the temperature is 70 ° C. At this time, another sample of the aqueous suspension is taken.

An analysis of the sample taken at the end of the vapor drive operation at constant temperature (90 ° C) shows that at this point the aqueous suspension contained an additional 250 mg of vinyl chloride per ml. kg of polyvinyl chloride.

An analysis of the second sample, taken after 20 minutes at the drop in water vapor at decreasing temperature, shows that the vinyl chloride content was reduced to 90 mg / kg polyvinyl chloride.

The initial staining of a crepe-like product prepared under the conditions described in Example 1 is very pale pink.

A comparison of Examples 1, 2 and 5 with Comparative Examples R1, R2 and R6 sufficiently demonstrates the efficiency of the residual vinyl chloride removal achieved in the process of the invention, both from the aqueous dispersion and from the dry polymer.

A comparison of Examples 1 and 7 shows that the completion of the treatment of the aqueous suspension with an in situ water vapor entrainment only allows excellent results to be achieved with less water vapor consumption and a total productivity gain.

EXAMPLE 8

This example relates to the treatment of an aqueous emulsion containing an ammonium salt of a fatty acid as an emulsifier.

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The aqueous emulsion is obtained by polymerization of vinyl chloride in aqueous emulsion at 70 ° C in the presence of ammonium stearate by degassing at 60 ° C under partial vacuum (absolute residual pressure of 350 mmHg). It is in the form of a stable emulsion with elemental particles of 0.1 µm.

The initial content of vinyl chloride in the degassed aqueous emulsion is 7 g / kg PVC. The polyvinyl chloride has a glass transition temperature of 88.5 ° C.

The degassed aqueous emulsion is treated as in Example 1 except that the entrainment is carried out at 95 ° C under a residual pressure of 634 mmHg over 15 minutes and that ammonia (10% aqueous solution) is introduced into an emulsion. sufficient quantity to keep the pH of the aqueous emulsion constant at its original value. In this way, destruction of the emulsifier is avoided.

After 15 minutes of treatment, the content of vinyl chloride in the aqueous emulsion is lower than 2 mg / kg polyvinyl chloride and the emulsion has maintained its stability.

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TABLE

Origin- Conditions for heat treatment- End-of-life-Ex. is the division of the watery suspension of teams of VC

team of VC? pension VC in the suspension dry in the suspension --- the pension polymer sion Tempera Tempera- Pressure Duration ** turn of turn of the vapor suspension g / kg PVC ° C ° C mmHg min. mg / kg PVC mg / kg 1 6.1 152 90 526 30 70 ± 2 2 6.1 152 100 760 15 25 ± 2 R3 6.1 152 80 355 30 780 200 R4 6.1 - 790 5 13.2 152 90 526 15 _2 <2 R6 13.2 - - - "- - 200 * VC: vinyl chloride PVC: polyvinyl chloride ** Duration: calculated from the time the suspension has reached the selected temperature.

Claims (5)

A process for removing residual vinyl chloride monomer from a vinyl chloride polymer containing more than 50 mole% of monomer units derived from vinyl chloride by heating to a temperature at least equal to the glass transition temperature of the polymer and treating with steam at this temperature, characterized by heating the aqueous polymer dispersion obtained by the polymerization directly to a temperature at least equal to the glass transition temperature of the polymer and subjecting it to entrapment with steam while maintaining a temperature which is at least is equal to the glass transition temperature of the polymer. Process according to claim 1, characterized in that the aqueous polymer dispersion is heated to a temperature below 140 ° C, preferably below 120 ° C. Process according to claim 1, characterized in that, by progressively lowering the pressure, a boiling in the aqueous dispersion is induced after it is subjected to vapor entrainment, gradually lowering the pressure, so as to maintain the boiling without heat supply from outside, as the temperature of the aqueous dispersion decreases. Process according to claim 3, characterized in that the cooking is terminated under progressively reduced pressure when the temperature of the dispersion has reached a value below the glass transition temperature of the polymer, preferably a value between 50 and 80 ° C. Process according to any one of claims 1-4 for removal of residual vinyl chloride monomer, characterized in that the monomer is removed from a vinyl chloride polymer obtained by polymerization in aqueous suspension.